Stable amyloid beta monomers and oligomers

ABSTRACT

The invention provides monomeric and oligomeric amyloid beta peptide isomers that are resistant towards fibrillogenesis and their use as screening reagents or antigens in methods and pharmaceutical preparations for the treatment of Alzheimer&#39;s disease and other conditions related to protein misfolding. The invention further provides transgenic animals expressing modified amyloid precursor proteins or amyloid beta peptides.

FIELD OF THE INVENTION

The invention relates to monomeric and oligomeric amyloid beta peptidesthat are resistant towards fibrillogenesis for use as antigens orscreening reagents for the treatment of Alzheimer's disease and otherconditions related to protein misfolding.

BACKGROUND OF THE INVENTION

Alzheimer's disease (AD) is an age-related neurodegenerative diseasecharacterized by the accumulation of insoluble amyloid deposits of theamyloid-β (Aβ) protein generated by the cleavage of the amyloidprecursor protein (APP) (Blennow, K., et al., Lancet 368: 387-403(2006)). AD is one of several diseases caused by protein misfolding,which includes over 22 other known ailments such as Parkinson's diseaseand Type II diabetes (Dobson, C. M., Protein Pept. Lett. 13: 219-227(2006)). There are at present no effective treatments against AD andmost of the other protein misfolding diseases, partly owing to the factthat it is still unclear if and how these deposits are toxic. In fact,focus has recently shifted away from the insoluble amyloid deposits,which previously were believed to be the cause of AD, towards soluble Aβoligomeric species as the toxic agent (Haass, C. and Selkoe D. J.,Nature Reviews Mol. Cell. Biol. 8: 101-112 (2007)). In addition,conformation-dependent antibodies raised against Aβ oligomers (e.g. thepolyclonal A11 antibody) have shown to be reactive towards otheroligomeric species in addition to Aβ, thus suggesting that these toxicstructures are generic in the protein misfolding diseases althoughdifferent proteins are involved (Glabe, C. G., Trends Biochem. Sci. 29:542-547 (2004)). Much effort is therefore currently being focusedworldwide on the isolation and characterization of such solubleoligomers for drug-screening and immunogenic purposes. However, bothmonomeric and oligomeric Aβ proteins have a high tendency to aggregatefurther into fibrils. This fibrillation is a spontaneousnucleation-dependent polymerization reaction for which the rate issensitive to peptide concentration (Lomakin, A., et al., Proc. Natl.Acad. Sci. USA 93: 1125-1129 (1996)). In consequence, fibrillogenesisseriously limits the longevity of the oligomeric preparations, and alsothe concentrations at which they can be kept.

The wild-type Aβ protein implicated in AD is a very fibrillation pronepeptide at concentrations >100 μM. At lower concentrations (>20 μM to<100 μM) said protein has a tendency to slowly oligomerize (oligomersare soluble structures containing a plurality of monomers) prior toproceeding into the inert fibrillar state. This oligomeric state hasrecently been implicated as a neurotoxic agent and, therefore, as thetoxic species involved in AD (Haass, C., and Selkoe, D. J., NatureReviews Mol. Cell. Biol. 8: 101-112 (2007)). The prevalent method forproducing these toxic structures at physiological conditions involveincubating peptide solutions at 20-100 μM and 4° C. in cell culturemedium (F-12) or in buffered salt solutions for several days (Lambert,M. P., et al., Proc. Natl. Acad. Sci. USA. 95: 6448-6453 (1998); Stine,W. B., et al., J. Biol. Chem. 278: 11612-11622 (2003)). Becausenucleation and elongation rates are strongly dependent on peptideconcentration (Lomakin, A., et al., Proc. Natl. Acad. Sci. USA 93:1125-1129 (1996)), any increase in the concentration of peptide above100 μM will be detrimental to the stability of the oligomer preparation.In the classical view of amyloid fibril assembly, the nuclei triggeringpolymerization and the oligomeric structures are even believed to be thesame species. Oligomer preparations therefore have a very limited andunpredictable (aggregation nucleation is a spontaneous event) lifespanonce they are formed. In dilute solutions (20 μM to 25 μM) at 4° C.oligomer preparations of Aβ(42) are typically stable only for 24 h, andthose of Aβ(40) for maybe a week. In less dilute solutions, >100 μM, theinsolubility of the Aβ protein decreases dramatically. The fibrillationprocess is thus a serious drawback when, as in screening assays formedicaments, stable proteins are required at relatively highconcentration.

The aim of the present invention is to overcome these problems byproviding engineered Aβ peptides that form stabile oligomers.

There are no previous reports of stabilized Aβ hairpin structures and noreports of Aβ peptides containing only the A21C/A30C disulphide withoutadditional complicating cysteines. US 2006/0018918 discloses Aβ isomersbased on a consideration of the Aβ primary structure alone with multiplecysteine replacement of all Ser and Ala in general with the intent ofstabilizing non-native conformations of Aβ to be used as vaccines. TheA21C and A30C mutations are obtained together with three or fouradditional cysteine mutants at position Ala2, Ser8, Ser26, and Ala42.Oxidation of these Aβ isomers produces a mixture of 15 possible isoformswith different intramolecular disulphide bonds where only three of the15 possible isoforms will contain an A21C/A30C disulphide bond, butalways in combination with additional disulphide bonds.

Previous Aβ peptide disulfide mutants reported in the literature areL17C/L34C, L17C/M35C, and L17C/V36C (Shivaprasad, S. and Wetzel, R.,Biochemistry 43: 153-15317 (2004)). These mutations are allnon-conservative replacements. Furthermore, these mutants were madespecifically to investigate the proximity of Leu17 to Leu34, Met35, andVa136 in the fibril structure. Shivaprasad and Wetzel present data thatdemonstrate that these three mutants undergo fibrillogenesis with lagtimes that are nearly identical for the reduced (with the mutatedresidues as cysteines) and oxidized (with the mutated residues ascystines) mutants. Other disulphide mutants, which are alsonon-conservative replacements, have been published by the same authors,namely V18C/L34C, F19C/A30C, F19C/I32C, and F19C/L34C (Wetzel, R., etal., Biochemistry 46: 1-10 (2007)). These mutants were found to behavesimilarly to the L17C/L34C, L17C/M35C, and L17C/V36C mutants. Hence, allpreviously published disulphide mutants of the Aβ protein have beendemonstrated to readily aggregate into fibrils of at least similarstability to the fibrils obtained from wild-type peptide. These oxidizedderivatives of the L17C/L34C, L17C/M35C, L17C/V36C, V18C/L34C,F19C/A30C, F19C/I32C, and F19C/L34C mutants all fibrillate because theyare incompatible with the hairpin structure presented in Hoyer et al.(Hoyer, W. et al., Proc. Natl. Acad. Sci. USA. 105: 5099-5104 (2008))whereas they are compatible with current models of Aβ-peptide fibrilstructures where the two β-strands pack against each other (see e.g.Petkova, A. T., et al., Biochemistry 45: 498-512).

Aβ oligomer preparations are described in WO 2007/005358, WO2007/005359, WO 2007/142320 and WO 2004/067561. These described oligomerpreparations have been obtained by using intermolecular crosslinkers (WO2007/005358), non-physiological pH (pH 9 in WO 2007/005359) and/oradditives (40% glycerol or TFE were claimed to stabilize oligomers at37° C. in WO 2007/005359; GM1 ganglioside in WO 2007/142320; and SDS inWO 2004/067561).

SUMMARY OF THE INVENTION

The present invention provides covalently constrained monomeric andoligomeric Aβ A21C/A30C peptides that are resistant towardsfibrillogenesis. The peptides according to the invention retain most ofits wild-type like properties as probed by size exclusion chromatography(SEC), far-UV CD (indicative of coil structure), and NMR (alsocoil-like). Since the fibrillation step is blocked, the Aβ peptidesaccording to the invention populate several low molecular-weightoligomers with coil-structure (eluting from SEC as up to approximately42 kDa proteins), as well as high molecular-weight oligomers withβ-structure (eluting from SEC as approximately 75-85 kDa and 170 kDaproteins) upon concentration. Furthermore, wild-type-like protofibrillarstructures with dimensions of 6 nm width and typically 30 nm in lengthare obtained by heating concentrated protein solutions. The highmolecular-weight oligomers with β-structure are recognized by the A11polyclonal antibody.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides monomeric and oligomeric Aβ peptides withan unprecedented resistance towards fibrillogenesis.

In order to prevent aggregation side-reaction from depleting the pool ofconcentrated monomeric and oligomeric Aβ peptide, the present inventorshave designed covalently constrained Aβ peptides. The Aβ peptidesaccording to the invention comprise amino acids replacements atpositions 21 and 30 in the wild-type Aβ(40) (SEQ ID NO: 1) or Aβ(42)(SEQ ID NO: 2) peptides with cysteines, resulting in intra-molecularlydisulphide bonded Aβ(40) A21C/A30C (SEQ ID NO: 3) and Aβ(42) A21C/A30C(SEQ ID NO: 4) peptides (FIG. 1A). These amino acids replacementseffectively block the conformational switch that triggers amyloidosis,one hypothesis of this mechanism is schematically depicted in FIG. 1B,and allows for the population of several oligomeric states, includingthe high molecular-weight oligomers with β-structure that are consideredto be the toxic species. The Aβ peptides according to the invention formpotentially toxic oligomeric states that are similar to those obtainedfor the wild-type Aβ peptide, having a far-UV circular dichroism (CD)spectrum identical to wild-type oligomers, and they also bind the A11antibody, and form protofibrils with wild-type-like morphology. Thenon-oligomeric coil-like state of the Aβ peptides according to theinvention also remain wild-type-like as probed by size SEC, far-UV CDspectroscopy, and ¹⁵N heteronuclear single quantum coherence (HSQC)nuclear magnetic resonance (NMR) spectroscopy.

Because of the constraining disulphide bond present in the peptidesaccording to the invention, other methods to stabilize oligomers are notneeded, including (i) non-physiological (elevated) pH to increaserepulsion of the peptides, (ii) using additives including2,2,2-trifluoroethanol (TFE), sodium dodecyl sulphate (SDS), glycerol,or cell membrane glycolipids in the form of GM1 gangliosides, to induceoligomeric structure, (iii) adding anions to screen charges thusincreasing solubility, and (iv) using an intermolecular cross-link thatcovalently stabilizes the oligomers once they are prepared by one orseveral of alternatives (i)-(iii). The Aβ peptides according to theinvention are in fact the first examples of Aβ peptides forming stableoligomers under physiological conditions, this being obtained byarresting the fibrillation step. Furthermore, long incubation times andunpredictable outcomes of oligomeric preparations are avoided, as the Aβpeptides according to the invention can be purified and rapidlyconcentrated into the oligomeric states.

In one aspect, the present invention provides peptides comprising theamino acid sequence LVFFC (SEQ ID NO: 13) corresponding to amino acids17 to 21 of SEQ ID NO: 4, and the amino acid sequence CIIGLMV (SEQ IDNO: 14) corresponding to amino acids 30 to 36 of SEQ ID NO: 4. Thepeptide according to the invention further comprises a disulphide bondbetween the amino acids corresponding to Cys21 and Cys30 in SEQ ID NO:4. In one preferred aspect of the invention, said peptide is obtained bylinking the peptide comprising the amino acid sequence LVFFC SEQ ID NO:13) to the peptide comprising the amino acid sequence CIIGLMV (SEQ IDNO: 14) by an inter-molecular disulphide bond. In a more preferredaspect of the invention, said two peptides are constituents of a singlepeptide chain with an intramolecular disulphide bond between Cys21 andCys30. Furthermore, the amino acid sequence LVFFC (SEQ ID NO: 13) isfused to the amino acid sequence CIIGLMV (SEQ ID NO: 14) by a peptidecomprising 4 to 20 amino acids, such as 6 to 16, preferably 7 to 12, oreven more preferably 8 to 10 amino acids, such as 9 amino acids. Thepresent invention further provides variants of said peptides wherein oneor more, such as one, two, three, four, five, or preferably one or two,amino acids have been replaced by conservative substitution, one ormore, such as one, two, three, four, five, or preferably one or two,amino acids have been deleted, or one or more, such as one, two, three,four, five, or preferably one or two, amino acids have been inserted.

Preferably, the fusion peptide comprises the amino acid sequenceEDVGSNKG (SEQ ID NO: 15) corresponding to amino acids 22 to 29 of SEQ IDNO: 4. Accordingly, the peptides according to the invention preferablycomprise the amino acid sequence LVFFCEDVGSNKGCIIGLMV (SEQ ID NO: 16)corresponding to amino acids 17 to 36 of SEQ ID NO: 4, and variants ofsaid peptides wherein one or more, such as one, two, three, four, five,or preferably one or two, amino acids have been replaced by conservativesubstitution, one or more, such as one, two, three, four, five, orpreferably one or two, amino acids have been deleted, or one or more,such as one, two, three, four, five, or preferably one or two, aminoacids have been inserted.

Even more preferably, the peptides according to the invention comprisethe amino acid sequence 1 to 40 of SEQ ID NO: 4, the full amino acidsequence SEQ ID NO: 4, and variants of said peptides wherein one ormore, such as one, two, three, four, five, or preferably one or two,amino acids have been replaced by conservative substitution, one ormore, such as one, two, three, four, five, or preferably one or two,amino acids have been deleted, or one or more, such as one, two, three,four, five, or preferably one or two, amino acids have been inserted.

The peptides according to the invention can further comprise a glycinesubstitution at the position corresponding to amino acid 22 (SEQ ID NO:5), or a glutamine substitution at the position corresponding to aminoacid 22 (SEQ ID NO: 6), or a lysine substitution at the positioncorresponding to amino acid 22 (SEQ ID NO: 8). The peptides according tothe invention can also comprise an asparagine substitution at theposition corresponding to amino acid 23 (SEQ ID NO: 7). The peptidesaccording to the invention can also comprise a methionine sulphoxideresidue at the position corresponding to amino acid 35 in SEQ ID NO: 4.

Furthermore, the peptides according to the invention can comprise (a)one or two terminal amino acids being maleimidated; (b) one or twoterminal amino acids being cysteinylated; (c), the carboxy terminal endof the peptide has been amidated, i.e. the free COOH at the carboxyterminal has been transformed into CONH₂; and/or the amino terminal endof the peptide has been acetylated, i.e. the free NH₂ group at the aminoterminal has been transformed into the amide CH₃CONH—(AcNH—). Thepeptides according to the invention can be prepared by chemicalsynthesis or be prepared by recombinant DNA technology.

By conservative substitution is meant substitution of one amino acidswith an amino acid having similar properties with regard to polarity andhydrophobicity. Examples of groups of amino acids having such similarproperties are provided in Table 1. Conservative substitution can alsoinclude substitution with a non-standard amino acid. Non-standard aminoacids include, but are not limited to, the examples within brackets inTable 1, where PL is pyrrolysine, DA is dehydroalanine, NL is norleucin,SC is selenocysteine, HC is homocysteine, CL is citrulline, and OR isornithine.

TABLE 1 Hydrophobic A V L I P F W M (apolar) [PL DA NL] PolarG S T C Y N Q (uncharged) (SC HC CL] PolarD E K R H  Positively   H K R [OR] (charged) [OR]       charged           Negatively   E D            charged

Preferably, conservative substitutions and amino acid insertionsaccording to the invention do not allow for insertion of furthercysteines in addition to the cysteines corresponding to Cys21 and Cys30in SEQ ID NO: 4.

Preferably, conservative substitutions and amino acid deletionsaccording to the invention do not allow for replacement or deletion ofthe cysteines corresponding to Cys21 and Cys30 in SEQ ID NO: 4.

Most preferably, the Aβ peptides according to the invention alwayscomprise exactly two cysteines, i.e. the cysteines corresponding toCys21 and Cys30 in SEQ ID NO: 4.

The peptides according to the invention are substantially resistanttowards fibrillogenesis. By being resistant to fibrillogenesis is meantthat said peptides do not form fibrils under near-physiologicalconditions where wild type Aβ peptide fibrillate, as demonstrated byseveral of the examples presented herein. In one aspect the presentinvention provides soluble oligomers consisting of peptides according tothe invention.

In another aspect the present invention provides an antibodyspecifically reacting with a peptide according to the invention. Thisantibody can be a monoclonal antibody. In another aspect the presentinvention provides an antibody fragment specifically reacting with apeptide according to the invention. The antibody fragment can be a Fabfragment, a (Fab)₂ fragment, a single chain Fab fragment, a single chainFv fragment, or a single chain Fv dimer. In yet another aspect thepresent invention provides a protein binder specifically reacting with apeptide according to the invention. Said protein binders include, butare not limited to, derivatives of the anticalin, Affibody, FNfn10,neocarzinostatin, ankyrin repeat protein, PDH finger, CDR3 grafted greenfluorescent protein, and E. coli periplasmic binding protein scaffolds.

In another aspect the present invention provides methods for theidentification of a compound suitable for the treatment of a diseasecaused by, or related to, the deposition of protein fibrils or amyloids,said method comprising the use of a peptide according to the invention.The disease can be an amyloidosis, such as an amyloid neuropathy orcerebral amyloid angiopathy, a prion disease such as Creuztfeldt-Jakobdisease, bovine spongiform encephalopathy, or scrapie, Parkinson'sdisease, or Alzheimer's disease. Preferably the disease is Alzheimer'sdisease. The method for the identification of a compound suitable forthe treatment of a disease caused or related to the deposition ofprotein fibrils or amyloids, can comprise:

-   -   a) providing a test compound,    -   b) contacting said test compound with a peptide according to the        invention,    -   c) determining if the test compound binds to the peptide and/or        inhibits the formation of peptide oligomers,    -   d) identifying said compound as suitable for the treatment of a        disease caused or related to the deposition of protein fibrils        or amyloids disease.        The disease can be an amyloidosis, such as an amyloid neuropathy        or cerebral amyloid angiopathy, a prion disease such as        Creuztfeldt-Jakob disease, bovine spongiform encephalopathy, or        scrapie, Parkinson's disease, or Alzheimer's disease. Preferably        the disease is Alzheimer's disease.

In another aspect the present invention provides methods for identifyingand/or selecting binding proteins, said method comprising the use of apeptide according to the invention. The binding molecule can be anantibody fragment, such as a Fab fragment, a (Fab)₂ fragment, a singlechain Fab fragment, a single chain Fv fragment, or a single chain Fvdimer. The binding molecule can also be engineered non-natural receptorderivatives, such as derivatives of the anticalin, Affibody, FNfn10,neocarzinostatin, ankyrin repeat protein, PDH finger, CDR3 grafted greenfluorescent protein, and E. coli periplasmic binding protein scaffolds.

In yet another aspect the present invention provides use of a peptideaccording to the invention in the preparation of pharmaceuticalcompositions intended for immunization for prophylactic or therapeutictreatment of a disease caused by, or related to, the deposition ofprotein fibrils or amyloids, optionally in combination with an adjuvant.The disease can be an amyloidosis, such as an amyloid neuropathy orcerebral amyloid angiopathy, a prion disease such as Creuztfeldt-Jakobdisease, bovine spongiform encephalopathy, or scrapie, Parkinson'sdisease, or Alzheimer's disease. Preferably the disease is Alzheimer'sdisease.

In yet another aspect the present invention provides pharmaceuticalpreparations comprising a therapeutically effective amount of a peptideaccording to the invention, optionally in combination with an adjuvant.In yet another aspect the present invention provides vaccines forimmunization of mammals, including humans, against a disease caused by,or related to, the deposition of protein fibrils or amyloids, comprisinga peptide according to the invention, optionally in combination with anadjuvant. The disease can be an amyloidosis, such as an amyloidneuropathy or cerebral amyloid angiopathy, a prion disease such asCreuztfeldt-Jakob disease, bovine spongiform encephalopathy, or scrapie,Parkinson's disease, or Alzheimer's disease. Preferably the disease isAlzheimer's disease.

In yet another aspect the present invention provides vaccines forimmunization of mammals, including humans, against a disease caused by,or related to, the deposition of protein fibrils or amyloids, comprisinga therapeutically effective amount of an antibody according to theinvention. The disease can be an amyloidosis, such as an amyloidneuropathy or cerebral amyloid angiopathy, a prion disease such asCreuztfeldt-Jakob disease, bovine spongiform encephalopathy, or scrapie,Parkinson's disease, or Alzheimer's disease. Preferably the disease isAlzheimer's disease.

In yet another aspect the present invention provides methods forprophylactic or therapeutic treatment of a mammal, including a humanbeing, suffering from a disease caused by, or related to, the depositionof protein fibrils or amyloids or facing the risk of developing adisease caused by, or related to, the deposition of protein fibrils oramyloids, whereby a therapeutically effective amount of a peptideaccording to the invention is administered to said mammal. The diseasecan be an amyloidosis, such as an amyloid neuropathy or cerebral amyloidangiopathy, a prion disease such as Creuztfeldt-Jakob disease, bovinespongiform encephalopathy, or scrapie, Parkinson's disease, orAlzheimer's disease. Preferably the disease is Alzheimer's disease.

In yet another aspect the present invention provides methods forprophylactic or therapeutic treatment of a mammal, including a humanbeing, suffering from a disease caused by, or related to, the depositionof protein fibrils or amyloids or facing the risk of developing adisease caused by, or related to, the deposition of protein fibrils oramyloids, whereby a therapeutically effective amount of an antibodyaccording to the invention is administered to said mammal.

DESCRIPTION OF THE FIGURES

FIG. 1A represents a drawing of the hairpin structure of the Aβ-peptide(Protein Data Bank accession no. 2OTK) with the introduced cysteines atpositions 21 and 30 and the disulphide bond linking these two positions(SEQ ID NO: 4). Leu17, Phe19, Cys21, Cys30, Ile32, Leu34, and Va136comprising the upwards facing side of the structure; and Val18, Phe20,Ile31, and Met35 comprising the downward facing side of the structure,this side also contains Gly33.

FIG. 1B represents a schematic depiction of a hypothetical aggregationscheme. In this hypothesis, the amfiphilic Aβ peptide spontaneouslyself-associates into micellar-like oligomeric structures of lowmolecular weight that are weakly stabilized by hydrophobic forces(coil-like oligomers). The β-hairpin is only transiently occupied, butis stable in the high molecular-weight oligomeric structures that formby hydrophobic stacking. The color coding of the hairpin is the same asin FIG. 1A. Fibrillogenesis occurs as a result of a conformationalswitch whereby the hydrogen bonds and hydrophobic van der Waals bonds inthe β-structured oligomer are broken and reformed in a much more stablefibril structure, wherein the two β-strands colored in orange are nowfacing each other in the fibril core. This conformational step isarrested by the disulphide bond connecting positions 21 and 30, thusallowing for the other states to become populated.

FIG. 2 represents fibrillation assays of the Aβ(40) peptide with andwithout the A21C/A30C replacements, as well as the Aβ(40) A21C/A30Cpeptide with a broken disulphide bond (in 5 mM tris(2-carboxyethyl)phosphine (TCEP)). Fibrillation was monitored by Thioflavin T (TFT)binding (10 μM) and the assays were carried out at 37° C. with shakingin phosphate buffer at pH 7.2.

FIGS. 3A-3E represent fibrillation assays of the Aβ(40) A21C/A30Cpeptide with formed and broken (by 10 mM TCEP) disulphide bonds at 30 μM(FIG. 3A), 50 μM (FIG. 3B), 150 μM (FIG. 3C) and 250 μM (FIG. 3D)peptide. FIG. 3E represents the baselines without peptide. Fibrillationwas monitored by TFT binding (10 μM) and the assays were carried out at37° C. with shaking in phosphate buffer at pH 7.2.

FIG. 4A represents the SEC elution profile of the Aβ(40) A21C/A30Cpeptide at low concentrations (approximately 25 μM), whereby saidpeptide elutes as approximately an 8 kDa protein.

FIG. 4B represents a far-UV CD spectrum of the Aβ(40) A21C/A30C peptideas a coil-like protein. This spectrum is virtually identical for allcoil-like preparations of said peptide.

FIG. 4C represents an SDS polyacrylamide gel electrophoresis (PAGE)analysis of the Aβ(40) A21C/A30C peptide with formed (−TCEP) and broken(+TCEP) disulphide bonds.

FIG. 4D represents a ¹⁵N-HSQC NMR spectrum of 50 μM Aβ(40) A21C/A30Cpeptide.

FIG. 5 represents an SEC elution profile of a concentrated solution (atapproximately 1.8 mM) of Aβ(40) A21C/A30C peptide that was denatured in6M guanidinium chloride (GdmCl) prior to SEC. According to far-UV CDspectroscopy, the high molecular-weight fractions all have β structure,and the low molecular-weight fractions all have coil structure.

FIG. 6A represents an SEC elution profile of a concentrated solution (atapproximately 1.5-1.8 mM) of Aβ(40) A21C/A30C peptide that was denaturedin 6M GdmCl prior to SEC.

In FIG. 6B the same solution was pre-treated with β-mercaptoethanolprior to SEC on the same column and in the same buffer supplemented with5 mM TCEP to keep the cysteines reduced. The peptide elutes in the voidunder these conditions (as >100 kDa proteins).

FIG. 6C represents the SEC elution profile of the top fraction in FIG.6A after 4 days at 4° C. and a concentration of 200 μM.

FIG. 6D represents the SEC elution profile of the peak centered around10.8 ml in FIG. 6A concentrated to approximately 1 mM.

FIG. 7A represents an SEC elution profile of a concentrated solution (atapproximately 1.7 mM) of Aβ(40) A21C/A30C peptide that was denatured in6M GdmCl prior to SEC. The peptide elutes as high molecular-weightoligomers with β structure, and as low molecular-weight oligomers andmonomeric protein with coil structure.

FIG. 7B represents a far-UV CD spectrum of 250 μM Aβ(40) A21C/A30Cpeptide as a β-structured peptide oligomer. This spectrum is virtuallyidentical for all preparations of said peptide with β structure,including protofibrillar states.

FIG. 8 represents a fibrillation assay of the Aβ(40) A21C/A30C peptideas β-structured oligomer with and without (in 10 mM TCEP) theconstraining disulphide bond. Fibrillation was monitored by TFT binding(10 μM) and the assay was carried out at 37° C. with shaking inphosphate buffer at pH 7.2.

FIG. 9 represents an SEC elution profile of approximately a 168 μMsolution of Aβ(40) A21C/A30C peptide as high molecular-weightβ-structured oligomer. The insets below the two peaks represent dotblots of the fractions (evenly distributed throughout the peaks) assayedfor A11 antibody binding capability.

FIG. 10A represents an SEC elution profile of a concentrated solution(at approximately 800 μM) of Aβ(40) A21C/A30C peptide denatured in 6 MGdmCl prior to SEC. The inset below the profile represents a dot blot ofthe fractions (evenly distributed throughout the peaks) assayed for A11antibody binding capability.

FIG. 10B represents an SEC elution profile of the first fraction in FIG.10A that had the highest A11 affinity. The inset below the profilerepresents a dot blot of the fractions (evenly distributed throughoutthe peaks) assayed for A11 antibody binding capability.

FIG. 11A represents far-UV CD spectra of the Aβ(40) A21C/A30C peptide asa coil-like protein at high concentration (1.1 mM), and itstransformation to β-structured protein after thermal denaturation from20° C. to 80° C. and back to 20° C. with a temperature slope of 2° C.min⁻¹.

FIG. 11B shows the thermal denaturation profile described in FIG. 11A asmonitored at 220 nm. There was no amorphous or fibrillar aggregationpresent in this sample neither before nor after thermal denaturation.

FIG. 12A represents transmission electron microscope (TEM) images of anegatively stained (uranyl acetate) solution of coil-like oligomers ofAβ(40) A21C/A30C peptide, which was approximately 500 μM at the time ofgrid application. The spherical structures are 15-35 nm in diameter.

FIG. 12B represents TEM images of the same solution as in FIG. 12Aincubated at 60° C. for 20 min prior to grid application. Theprotofibrillar-like structures are 6-7 nm in width and on averageapproximately 36 nm in length.

FIG. 12C represents TEM images of a similarly negatively stainedsolution of approximately 150 μM Aβ(40) A21C/A30C as a β-structured highmolecular-weight oligomer. The spherical structures are similar to thosein FIG. 12A, having diameters of 15-35 nm. The scale bar is 200 nm inthese figures.

FIG. 13 represents a fibrillation assay of the Aβ(42) A21C/A30C peptidewith formed and broken (by 10 mM TCEP) disulphide bonds at 117 μM.Fibrillation was monitored by TFT binding (10 μM) and the assays werecarried out at 37° C. with shaking in phosphate buffer at pH 7.2.

FIG. 14 represents an SEC elution profile of a concentrated solution ofAβ(42) A21C/A30C peptide that was denatured in 6M GdmCl prior to SEC.The insets below the chromatogram represents dot blots of the fractions(the amount of sample applied to the membrane was normalized accordingto the absorbance at 280 nm) assayed for A11 antibody bindingcapability.

FIG. 15A represents a time-dependent assay of a 80 kDa Aβ(40) A21C/A30CSEC fraction that was concentrated to 20 μM and incubated at 37° C. Timepoints were taken and assayed for the presence of the A11 epitope.

FIG. 15B represents a time-dependent assay of a 10 kDa Aβ(42) A21C/A30CSEC fraction that was concentrated to 50 μM and incubated at 37° C. Timepoints were taken and assayed for the presence of the A11 epitope.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides covalently constrained Aβ peptides havingA21C/A30C disulphide replacements that are resistant towardsfibrillogenesis, and their use in the screening for therapeutic agentsagainst toxic oligomeric species involved in Alzheimer's and otherprotein misfolding diseases. The peptides according to the invention canalso be used directly as immunogens to raise an immune response againsttoxic forms of the Aβ peptide. The invention is an example of the use ofrational peptide engineering to modify peptide properties to suit newneeds; the need here being the desire to arrest Aβ peptidefibrillogenesis at the toxic oligomeric level so that these species canbe isolated and utilized. The examples presented below demonstrate thatthe peptides according to the invention share many properties of thewild-type Aβ peptide except the high tendency of wild-type Aβ peptide tofibrillate.

Herein, the term oligomer is used as a collective term for solubleaggregates containing a plurality of Aβ-peptide monomers. These monomersare peptide fragments derived from the APP (Swiss-Prot entry P05067),and are commonly comprised of 39 to 43 amino acids. The full lengthsequence is described in Kang, J., et al., Nature 325: 773-776 (1987).The invention provides stabilized peptides corresponding to all of the39 to 43 residue long derivatives of the APP including the two mostfrequently encountered derivatives, Aβ(40) and Aβ(42), for which thesequences are:

Human Aβ(40): (SEQ ID NO: 1) DAEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGLMVGGVVHuman Aβ(42): (SEQ ID NO: 2) DAEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGLMVGGVVIA

Accordingly, one preferred aspect of the invention provides peptidescomprising the following sequences:

Human Aβ(40) A21C/A30C: (SEQ ID NO: 3)DAEFRHDSGYEVHHQKLVFFCEDVGSNKGCIIGLMVGGVV Human Aβ(42) A21C/A30C:(SEQ ID NO: 4) DAEFRHDSGYEVHHQKLVFFCEDVGSNKGCIIGLMVGGVVIA

Single-letter symbols and three-letter symbols are exchangeable used todenote the amino acids. These symbols, which are well known to manskilled in the art, have the following meaning: A=Ala=alanine,C=Cys=cysteine, D=Asp=aspartic acid, E=Glu=glutamic acid,F=Phe=phenylalanine, G=Gly=glycine, H=His=histidine, I=Ile=isoleucine,K=Lys=lysine, L=Leu=leucine, M=Met=methionine, N=Asn=asparagine,P=Pro=proline, Q=Gln=glutamine, R=Arg=arginine, S=Ser=serine,T=Thr=threonine, V=Val=valine, W=Trp=tryptophan, and Y=Tyr=tyrosine.

The peptides according to the invention was specifically designed inorder to prevent fibrillation of the Aβ peptides derived from the APP.Design of the peptides according to the invention (FIG. 1A) is based onthe recently determined NMR structure (Protein Data Bank accession no.2OTK) of the Aβ peptide (Hoyer, W., et al., Proc. Natl. Acad. Sci. USA.105: 5099-5104 (2008)). The constraining disulfide bond present in thepeptides of the invention connecting residues 21 and 30 stabilizes thisβ-hairpin structure once it is formed, and further inhibits theconformational switch that triggers amyloidosis, (one hypothesis of sucha mechanism is schematically depicted in FIG. 1B. In consequence, the Aβpeptides according to the invention are very resistant towardsfibrillogenesis even at high concentrations unless the disulphide bondis reduced (disrupted) to form thiols (RSH) with e.g. dithiothreitol(DTT), β-mercaptoethanol, or TCEP, under physiological conditions, underwhich the intramolecular covalent link between Cys21 and Cys30 isstable. Further oxidation of the sulphides to sulphenic (RSOH),sulphinic (RSO₂H), and sulphonic (RSO₃H) acid with e.g. reactive oxygenspecies (such as H₂O₂) or similar harsh treatments undernon-physiological conditions, is likely to trigger fibrillogenesis ofthis peptide.

Positions 21 and 30 of the Aβ peptide (as defined in e.g. SEQ ID NO: 2)are the only two residues that can be covalently linked by a disulphidebond without disrupting said β-hairpin structure. Two geometricalcriteria must be fulfilled for such a successful incorporation of astabilizing disulphide bond, namely (I) that the C_(α)-C_(α) distancebetween the positions that are to be linked are within 4.4-6.8 Å, and(ii) that the C_(β)-C_(β) distance between the same two residues arewithin 3.45-4.50 Å (Clarke, J., and Fersht, A., Biochemistry 32:4322-4329 (1993)). In addition, similar peptide engineering is likely tobe applicable on all Aβ peptide derivatives that are capable of formingsaid hairpin structure either as the transiently occupied monomericspecies, or as the stable oligomeric species. Accordingly, the inventionis not limited to A21C/A30C Aβ peptides corresponding to the 39 to 43residue long Aβ peptides with wild-type sequences, but also includesA21C/A30C Aβ peptides corresponding to naturally occurring Aβ-peptidemutants that lead to an increased incidence of familial AD. Thesemutations can be found in any of the 39-43 long Aβ-peptide derivativesof the APP. The invention thus includes, but is not limited to A21C/A30CAβ peptides carrying, the E22G arctic mutation (SEQ ID NO: 5), the E22QDutch mutation (SEQ ID NO: 6), the D23N Iowa mutation (SEQ ID NO: 7),and the Italian E22K mutation (SEQ ID NO: 8), and other familialmutations that are likely to appear. Furthermore, the hairpin structurein said sequences containing a methionine sulphoxide or sulphone atposition 35 can be stabilized by the invention. This methionine isbelieved by many to be essential for toxicity (see e.g. Butterfield, D.A., and Kanski, J., Peptides 23: 1299-1309 (2002)).

Other A21C/A30C Aβ peptides comprising one or more conservativemutations or otherwise made conservative substitutions, alterations,insertions or truncations (e.g. the 9-42 truncated derivative; SEQ IDNO: 9) of the above mentioned sequences, including non-standard aminoacids and end terminal modifications, such as cysteinylation,maleimidation, acetylation, and amidation, are also part of theinvention, and are fibrillization-resistant. Amino acids outside of thef3-hairpin structure, i.e. residues 1 to 16 and 37 to 40 of SEQ ID NO:3, and 1 to 16 and 37 to 42 of SEQ ID NO: 4, are more resilient tosubstitution and, therefore, even non-conservative replacements in theseregions are tolerated without seriously affecting the function of theinvention. Other modified A21C/A30C Aβ peptides that are part of theinvention include, but is not limited to, peptides wherein one or moreconsecutive stretches of Aβ-peptide sequence have been inverted, such asthe 1-8 N-terminal inverted derivative (SEQ ID NO: 10). The Aβ peptidesaccording to the invention can be produced by chemical synthesisfollowed by separation of oxidized peptides (sulphides) from reducedpeptides (thiols) by reverse-phase high-performance liquidchromatography (HPLC) as previously described for other disulfidederivatives of this peptide (Shivaprasad, S. and Wetzel, R.,Biochemistry 43: 153-15317 (2004)). Chemical synthesis is the mostcommon way to produce the Aβ peptides. Since these AO peptides areusually lyophilized, it may first be necessary to pretreat the peptideswith 100% 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) to removepre-existing aggregates prior to characterization as discussedpreviously (Stine, W. B. et al. J. Biol. Chem. 278: 11612-11622 (2003)).

Heterologous expression of the Aβ peptides according to the invention inEscherichia coli is an alternative to chemical synthesis, but requiresfor the peptides to be expressed as fusion proteins to aid itssolubility (see e.g. Lee, E. K., et al., Prot. Expr. Purif 40:183-189(2005)). A method of choice for cost-efficient production of Aβ(40) andAβ(42) is to co-express these peptides with an affinity binding protein(Gronwall, C., et al., J. Biotechnol. 128: 162-183 (2007)) in E. coli.In brief, the peptides are expressed concomitantly with the bindingprotein, thus forming a soluble complex that protects the Aβ peptideagainst aggregation and proteolysis. The binding protein contains ahexa-histidine tag to aid in the purification of the complex usingimmobilized metal affinity chromatography (IMAC). Once purified, thecomplex can be broken using GdmCl, and the released Aβ peptide can beseparated from the affinity binding protein again using IMAC underdenaturing conditions. The GdmCl can then be dialyzed away or removedusing SEC. This co-expression protocol does not involve lyophilization,and enables for direct selection of the peptides during expression. As aresult from expression in E. coli, all the peptides used in the datapresented below contain an extra N-terminal methionine. We here denotethis position as −1, so that the amino acid sequence in the Aβ peptidesretain their original numbering. The extra methionine does not limit oraffect the property of the peptides, and similar results are obtainedwith wild-type Aβ and A21C/A30C Aβ peptides if synthesized and purifiedas previously described (Shivaprasad, S. and Wetzel, R., Biochemistry43: 153-15317 (2004)).

Aggregation assays of Aβ(40) A21C/A30C were carried out undernear-physiological conditions (50 mM K⁺ phosphate buffer, 50 mM NaCl, pH7.2) at various concentrations at 37° C. with shaking. These experimentsdemonstrate that this peptide is very resistant towards fibrillogenesis,and that the reduced (in the presence of 5 mM TCEP) fully fibrillatedspecies have TFT binding properties comparable to wild-type peptide(Example 1). Two features are evident when the same experiment iscarried out at higher concentrations of peptide (Example 2): First,there is a rapid initial increase in TFT fluorescence at concentrationsexceeding 100 μM that subsequently declines to similar levels in allsamples; Second, as the concentration of peptide increases, there isalso an increase in lag times preceding fibrillation in the reducedsamples. These two observations are features of the oligomerictendencies of this peptide: The rapid increase in fluorescence is aresult of the initial formation of protofibrillar-like species thatsubsequently dissociate as a new equilibrium is established, and theincreased lag-times are likely to be a result of the disulphide bondbeing more or less inaccessible in the coil-like oligomers that exist athigh concentration.

Apart from being resistant towards fibrillogenesis, the Aβ(40) A21C/A30Cpeptide resembles the wild-type peptide. CD and NMR spectra of the lowmolecular-weight fraction (eluting at approximately 8 kDa) obtained fromSEC demonstrate that said peptide forms coil-like structures that arevery similar to wild-type peptide (Example 3). At higher concentrationsseveral oligomeric species containing a plurality of monomers elute fromthe SEC runs. These frequently elute within the range of >8 to <42 kDa(Example 4), and they all have coil-like structures with CD spectraidentical to the peptide obtained in the low molecular-weight fraction.These coil-like oligomers are denoted low molecular-weight oligomers.However, the peptide is also capable of forming larger oligomers with βstructures. These structures elute from the SEC columns as proteins ofsizes of approximately 75-85 kDa and 170 kDa (Example 5). We hereindenote these β structured oligomers as high molecular-weight oligomers.In aggregation assays, these high molecular-weight oligomers areresistant towards fibrillogenesis. Even the presence of 10 mM TCEP couldnot nucleate polymerization of these oligomers at 30 μM peptideconcentration (Example 5). Some of these high molecular-weight oligomersalso bind the A11 antibody (Example 6), which is a polyclonal antibodyspecifically raised against Aβ peptide oligomers and is suggested to begeneric for disease-related oligomeric species (Kayed, R. et al. Science300: 486-489 (2003)).

Low molecular-weight oligomers of the peptides of the invention at highconcentration can be made to form protofibril-like structures rapidly bythe application of heat (60° C.) (Example 7). This step has not beenoptimized, and other elevated temperatures (approximately >20° C.) willhave the same effect, although a temperature below 80° C. is desirableto prevent irreversible covalent modifications of the peptide fromoccurring (including deamidation of Asn27 and Gln15, as well asoxidation of the Cys21-Cys30 disulphide bond and Met35). According toTEM, these protofibrils have an average dimension of approximately 6.7nm in width, and lengths in the order of approximately 36 nm (althoughthe lengths vary) (Example 8). The transition is accompanied by theformation of β structure, for which the CD spectrum is indistinguishablefrom the high molecular-weight oligomers above. These structures alsobind TFT. In aggregation assays, this structural transition thereforeresults in a rapid initial increase in fluorescence, which is followedby a gradual decrease when the protofibrils dissociate as a newequilibrium is established. This observation is consistent with the viewthat Aβ-peptide protofibrils are in equilibrium with monomeric speciesbut at the same time being fibril precursors (Walsh, D. M., et al., J.Biol. Chem. 274: 25945-25952 (1999)).

The longer peptides according to the invention, i.e. the Aβ(42)derivatives (Example 9) are also protected from fibrillization, and theoligomers thus obtained during SEC of Aβ(42) A21C/A30C have a similarsize distribution as the Aβ(40) A21C/A30C oligomers (Example 10). TheA11 epitope is present in the largest of the high molecular-weightoligomers with β structure that elute from the SEC column (Example 10).This A11-binding epitope is very stable once formed and only decreaseswith 0.1% per day when incubated at 37° C. over a two-month period(Example 11).

The peptides according to the invention, when used for the production ofcovalently stabilized oligomers, provides advantages over previousmethods in that these peptide's resistance towards fibrillogenesisenables such oligomers to be prepared under physiological conditions.These oligomers can then be studied at higher concentrations and/or forlonger times than previously possible, thus aiding considerably in drugscreening assays and immunization trials. High molecular-weightoligomers of the Aβ peptides according to the invention can be produced(e.g. as in example 5), concentrated, and then used in methods for theidentification of compounds potentially suitable for the treatment ofAD. Such a compound should bind to already formed toxic oligomers anddissolve them, and/or prevent their formation. There are severalbiophysical techniques that can be used to monitor the disruption ofpreexisting oligomeric structures or their non-existence including CD,NMR, isothermal titration calorimetry, or enzyme-linked immunosorbentassays using an antibody or binding protein specific for the oligomericor monomeric structures. Even the fluorophores 1-anilino-8-naphthalenesulfonate and TFT, which both fluoresces when bound to the oligomers,can be used in such an assay as the fluorescence is quenched uponoligomer dissociation. A person skilled in the art will realize thatthere are several other possibilities. In another embodiment of theinvention, the covalently stabilized monomers or oligomers may be usedfor the preparation of antigen formulations as immunogens for thegeneration of antibodies or other protein binders. Both active andpassive immunization against the hairpin structure here shown to bepresent in the potentially toxic oligomeric forms of AD may come intoquestion.

In yet another aspect the present invention provides non-humantransgenic animals expressing a nucleotide sequence encoding an amyloidprecursor protein (APP) comprising the A21C/A30C mutations. Preferably,the APP is a mutated human APP comprising the A21C/A30C mutations, mostpreferably the APP is the protein SEQ ID NO: 11. Preferably, thenucleotide sequence is a mutated human APP gene comprising the A21C/A30Cmutations. The non-human transgenic animal is preferably selected fromthe phylum Chordata (and Hemichordata), such as a vertebrate animal(which include cyclostomes, bony fish, sharks and rays, amphibians,reptiles, mammals, and birds). Preferably, the non-human transgenicanimal is a mammal, such as a mouse, rat, pig, rabbit, or guinea pig.Most preferably the non-human transgenic mammal is a mouse.

In another embodiment of the invention, non-human transgenic animals canbe used as model systems for AD pathogenesis by overexpressing only anAβ peptide according to the invention comprising the A21C/A30Creplacements. As no proteolytic processing of APP is required togenerate the toxic Aβ peptide in such a model organism, non-humantransgenic animals may not only be selected from within the chordates(and hemichordates), but may also include invertebrates which lackendogenous APP comprising the Aβ peptide (e.g. selected from any of thephyla Annelida, Arthropoda, Cnidaria, Echinodermata, Mollusca, Nematoda,Nematomorpha, Platyhelminthes, and Porifera). Invertebrate models of ADpathogenesis have the advantage of short life spans, low costs, smallsizes, and highly characterized genetics. Examples of such invertebratemodel systems include Drosophila melanogaster (Crowther, D. C. et al.(2005) Neuroscience 132: 123-135) and Caenorhabditis elegans (Link, C.D. (1995) Proc. Natl. Acad. Sci. USA 92: 9368-9372). Accordingly, in yetanother aspect the present invention provides non-human transgenicanimals expressing a nucleotide sequence encoding an Aβ peptideaccording to the invention comprising the A21C/A30C replacements.Preferably, the heterologously expressed Aβ peptide in such aninvertebrate model system is the peptide corresponding to SEQ ID NO: 3,SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8 SEQID NO: 9 and/or SEQ ID NO: 10.

In yet another aspect the present invention provides a nucleotidesequence encoding an APP comprising the A21C/A30C mutations. Preferablythe APP is a mutated human APP comprising the A21C/A30C mutations, mostpreferably the APP is the protein SEQ ID NO: 11. Preferably thenucleotide sequence is a mutated human APP gene comprising the A21C/A30Cmutations.

Another aspect of the present invention is an expression systemcomprising a nucleotide sequence encoding APP comprising the A21C/A30Cmutations. Yet another aspect of the present invention is a polypeptidewhich is an APP comprising the A21C/A30C mutations, preferably a mutatedhuman APP comprising the A21C/A30C mutations, most preferably thepolypeptide SEQ ID NO: 11.

The amino acid position 21 in the human Aβ peptide corresponds to theamino acid position 692 in human APP, and amino acid position 30 in thehuman Aβ peptide corresponds to the amino acid position 701 in humanAPP. The sequence of human APP can be found in Swiss-Prot entry P05067(SEQ ID NO: 12). Consequently, by an APP comprising the A21C/A30Cmutations is meant an APP wherein 0 the amino acid alanine in position692, as defined by the sequence of the human APP (SEQ ID NO: 12) or thecorresponding position in a homologous or heterologous APP, is replacedwith a cysteine, and ii) the amino acid alanine in position 701, asdefined by the sequence of the human APP (SEQ ID NO: 12) or thecorresponding position in a homologous or heterologous APP, is replacedwith a cysteine.

By a homologous or heterologous APP is meant an APP derived from ananimal, such as a vertebrate, as well as a synthetic, mutated, and/ornon naturally occurring APP, which has a sequence identity of at least80%, such as at least 90%, or at least 95% as compared to human APP (SEQID NO: 12). The percent identity between two amino acid sequences isdetermined as follows. First, an amino acid sequence is compared to, forexample, SEQ ID NO: 12 using the BLAST 2 Sequences (Bl2seq) program fromthe stand-alone version of BLASTZ containing BLASTN version 2.0.14 andBLASTP version 2.0.14. This stand-alone version of BLASTZ can beobtained from the U.S. government's National Center for BiotechnologyInformation web site at ncbi.nlm.nih.gov. Instructions explaining how touse the Bl2seq program can be found in the readme file accompanyingBLASTZ. Bl2seq performs a comparison between two amino acid sequencesusing the BLASTP algorithm. To compare two amino acid sequences, theoptions of Bl2seq are set as follows: −i is set to a file containing thefirst amino acid sequence to be compared (e.g., C:\seq1.txt); −j is setto a file containing the second amino acid sequence to be compared(e.g., C:\seq2.txt); −p is set to blastp; −o is set to any desired filename (e.g., C:\output.txt); and all other options are left at theirdefault setting. For example, the following command can be used togenerate an output file containing a comparison between two amino acidsequences: C:\Bl2seq −i c:\seq1.txt −j c:\seq2.txt −p blastp −oc:\output.txt. If the two compared sequences share homology, then thedesignated output file will present those regions of homology as alignedsequences. If the two compared sequences do not share homology, then thedesignated output file will not present aligned sequences. Once aligned,the number of matches is determined by counting the number of positionswhere an identical nucleotide or amino acid residue is presented in bothsequences. The percent identity is determined by dividing the number ofmatches by the length of the sequence set forth in an identifiedsequence followed by multiplying the resulting value by 100. Forexample, if a sequence is compared to the sequence set forth in SEQ IDNO: 12 (the length of the sequence set forth in SEQ ID NO: 12 is 770)and the number of matches is 693, then the sequence has a percentidentity of 90 (i.e., 693÷770×100=90) to the sequence set forth in SEQID NO: 12.

Production of Transgenic Animals

Methods for generating transgenic animals of the present invention arewell known in the art (see, generally, Gene Targeting: A PracticalApproach, Joyner, ed., Oxford University Press, Inc. (2000)). In oneembodiment, generation of the transgenic mouse may optionally involvedisruption of the murine APP genes and introduction of one or morecopies of the gene encoding a mutated human APP into the murine genome,preferably at the same location as the endogenous murine APP gene.

The transgenic non-human animals of the invention are preferablyproduced by introducing transgenes into the germline of the animal.Embryonic target cells at various developmental stages can be used tointroduce transgenes. Different methods are used depending on the stageof development of the embryonic target cell. The specific line(s) of anyanimal used to practice this invention are selected for general goodhealth, good embryo yields, good pronuclear visibility in the embryo,and good reproductive fitness. When transgenic mice are to be produced,strains such as C57BL/6 or C57BL/6×DBA/2 F1, or FVB lines are often used(obtained commercially from Charles River Labs, Boston, Mass., TheJackson Laboratory, Bar Harbor, Me., or Taconic Labs.).

Introduction of the transgene into the embryo can be accomplished by anymeans known in the art such as, for example, microinjection,electroporation, or lipofection. For example, the transgene can beintroduced into a mammal by microinjection of the construct into thepronuclei of the fertilized mammalian egg(s) to cause one or more copiesof the construct to be retained in the cells of the developingmammal(s). Following introduction of the transgene construct into thefertilized egg, the egg may be incubated in vitro for varying amounts oftime, or reimplanted into the surrogate host, or both. One common methodis to incubate the embryos in vitro for about 1-7 days, depending on thespecies, and then reimplant them into the surrogate host. Reimplantationis accomplished using standard methods. Usually, the surrogate host isanesthetized, and the embryos are inserted into the oviduct. The numberof embryos implanted into a particular host will vary by species, butwill usually be comparable to the number of off spring the speciesnaturally produces.

Retroviral infection can also be used to introduce transgenes intonon-human animals. The developing non-human embryo can be cultured invitro to the blastocyst stage. During this time, the blastomeres can betargets for retroviral infection (Jaenich, R. (1976) Proc. Natl. Acad.Sci. USA 73:1260-1264). Efficient infection of the blastomeres isobtained by enzymatic treatment to remove the zona pellucida(Manipulating the Mouse Embryo, Hogan eds., Cold Spring HarborLaboratory Press, Cold Spring Harbor, 1986). The viral vector systemused to introduce the transgene is typically a replication-defectiveretrovirus carrying the transgene (Jahner et al. (1985) Proc. Natl.Acad. Sci. USA 82:6927-6931; Van der Putten et al. (1985) Proc. Natl.Acad. Sci. USA 82:6148-6152). Transfection is easily and efficientlyobtained by culturing the blastomeres on a monolayer of virus-producingcells (Van der Putten, supra; Stewart et al. (1987) EMBO J. 6:383-388).Alternatively, infection can be performed at a later stage. Virus orvirus-producing cells can be injected into the blastocoele (Jahner etal. (1982) Nature 298:623-628). Most of the founders will be mosaic forthe transgene since incorporation occurs only in a subset of the cellswhich formed the transgenic non-human animal. Further, the founder maycontain various retroviral insertions of the transgene at differentpositions in the genome which generally will segregate in the offspring.In addition, it is also possible to introduce transgenes into the germline by intrauterine retroviral infection of the midgestation embryo(Jahner et al. (1982) supra).

A third type of target cell for transgene introduction is the embryonicstem (ES) cell. Transgenes can be efficiently introduced into the EScells by DNA transfection or by retrovirus-mediated transduction. Suchtransformed ES cells can thereafter be combined with blastocysts from anon-human animal. The ES cells thereafter colonize the embryo andcontribute to the germ line of the resulting chimeric animal.

In one embodiment of the invention, an endogenous APP gene in a nonhumanhost is functionally disrupted by homologous integration of aheterologous APP gene comprising the A21C/A30C mutations, such that theheterologous APP gene substantially replaces the endogenous APP gene,and preferably completely replaces the coding sequences of theendogenous APP gene. Preferably, the heterologous APP gene is linked, asa consequence of homologous integration, to regulatory sequences (e.g.,an enhancer/promoter) of the endogenous APP gene, respectively, so thatthe heterologous gene is expressed under the transcriptional control ofregulatory elements from the endogenous APP gene locus. Nonhuman hostswhich are homozygous for such replacement alleles may be producedaccording to methods described herein. Such homozygous nonhuman hostsgenerally will express a heterologous APP but do not express theendogenous APP protein. Usually, the expression pattern of theheterologous humanized APP gene will substantially mimic the expressionpattern of the endogenous APP gene, in the naturally-occurring(non-transgenic) nonhuman host.

For example, a transgenic mouse can be generated that has APP genesequences comprising the A21C/A30C mutations in place of endogenousmurine APP gene sequences and which are transcriptionally controlled byendogenous murine regulatory sequences. The APP gene sequencescomprising the A21C/A30C mutations generally will be expressed similarlyto the murine APP in naturally occurring non-transgenic mice.

Generally, a replacement-type targeting construct is employed forhomologous gene replacement. Double-crossover homologous recombinationbetween endogenous APP gene sequences of the targeting construct resultin targeted integration of the heterologous APP gene segments. Usually,the homology targeting regions of the transgene comprise sequences whichflank the endogenous APP gene segments, so that homologous recombinationresults in concomitant deletion of the endogenous APP, and homologousintegration of the heterologous APP gene segments. Substantially anentire endogenous APP gene may be replaced with a heterologous APP by asingle targeting event or by multiple targeting events (e.g., sequentialreplacement of individual exons). One or more selectable markers,usually in the form of positive or negative selection expressioncassettes, may be positioned in the targeting construct. It is usuallypreferred that selectable markers are located in intron regions of theheterologous replacement region.

Transgenic animals comprising transgene APP gene sequences comprisingthe A21C/A30C mutations can be crossed with other transgenic animals,generating transgenic animals comprising multiple transgenes. A mannerof preparation is to generate a series of mammals, each containing oneof the desired knockout constructs or transgenes. Such mammals are bredtogether through a series of crosses, backcrosses and selections, toultimately generate a single mammal containing all desired knockoutconstructs and/or transgenes, where the mammal is otherwise congenic(genetically identical) to the wild type except for the presence of theknockout(s) constructs and/or transgene(s). Typically, crossing andbackcrossing is accomplished by mating siblings or a parental strainwith an offspring, depending on the goal of each particular step in thebreeding process. In certain cases, it may be necessary to generate alarge number of offspring in order to generate a single offspring thatcontains each of the knockout constructs and/or transgenes in the properchromosomal location. In addition, it may be necessary to cross orbackcross over several generations to ultimately obtain the desiredgenotype.

EXAMPLES Example 1. Fibrillation Assay of Wild-Type Aβ(40) and Aβ(40)A21C/A30C

All concentration determinations were carried out spectrophotometricallyusing an extinction coefficient of 1424 cm⁻¹ M⁻¹ for the difference inabsorption at 280 nm and 300 nm. The buffered solution used in allexamples was 50 mM K⁺ phosphate, 50 mM NaCl, pH 7.2, unless otherwisestated. This buffer was also the storage buffer for the Aβ peptides. Allpeptide samples were prepared fresh, kept at 4° C., and used within 3-4days.

Fibrillation assays were carried out by monitoring the enhancedfluorescence of the dye TFT upon binding to the fibrils (Levine, H.,Methods Enzymol. 309: 274-284 (1999)). Fluorescence was recorded in96-well plates (Nunc) using a FLUOstar Optima reader (BMG) equipped with440 nm excitation and 480 nm emission filters. The peptide samples werekept at 30 μM and were supplemented with 10 μM TFT. In addition, 5 mMTCEP was added to one reference sample of the invention to assay theeffect of the formed disulphide bond. TCEP is a more stable reductantthan the more commonly used DTT at basic pH. It reduces the disulphidebonds to thiols, thus disrupting the constraining covalent bond of theinvention. Prior to measurements, plates were sealed with polyolefintape (Nunc) to prevent evaporation. The assay was carried out at 37° C.,and data points were recorded every 6 min with 2 min of orbital shaking(width 5 mm) preceding each measurement.

The result of this aggregation assay is presented in FIG. 2. Nucleationand fibrillation occurs rapidly in the wild-type sample. For theA21C/A30C peptide reduced with TCEP, the lag time is approximately 5 hlonger, but the TFT fluorescence of the fully fibrillated end state isidentical to the wild-type fibril. For the oxidized A21C/A30C peptide,on the other hand, there is no fibrillation occurring.

Example 2. Fibrillation Assays of Aβ(40) A21C/A30C at IncreasingConcentrations

The fibrillation assay was carried out as in example 1, except that theconcentration of peptide was also assayed at 50, 150, and 250 μM, andthe TCEP concentration was increased to 10 mM TCEP in the referencesamples. Also, data points were recorded every 15 min with 5 min oforbital shaking (width 5 mm) preceding each measurement. Dual sampleswere analyzed at each concentration, and the result is presented in FIG.3.

The result is similar to the result obtained in Example 1. However,there is a rapid increase in fluorescence for the samples with an intactdisulphide bond at the 150 and 250 μM concentrations, which then declineto similar levels with much slower rates. As demonstrated in Examples 7and 8 below, the coil-like oligomers can be made to formprotofibrillar-like structures when subjected to elevated temperatures,and these do bind TFT weakly thus giving rise to enhanced fluorescence.As time progresses, a new equilibrium is reached in these samples.Observations made by Walsh et al. for the wild-type Aβ peptide areapplicable to the data in FIG. 3 (Walsh, D. M., et al., J. Biol. Chem.274: 25945-25952 (1999)). They show data that suggest that theprotofibrils are fibril precursors, and that the competing rateconstants for protofibril dissociation and fibril formation are ofsimilar magnitude. But the conformational switch triggering fibrillationin wild-type Aβ Aβ peptide is inhibited in the Aβ A21C/A30C peptide,which is why no fibrils can be detected in these highly concentratedsamples unless the disulphide bond is broken. The slower increase influorescence in the 50 μM sample is likely due to the fact that theprotofibrils are not nucleated at this lower concentration, but that thestructural transition from coil- to β-structure nevertheless occurs as aresult of the higher temperature and/or agitation of the sample. It hasindeed been shown previously that that binding of TFT is dependent onthe presence of β-sheet structure (DeLellis, R. A., et al., J.Histochem. Cytochem. 16: 663-665 (1968)).

Another feature of the increased peptide concentration is the increasein lag times preceding nucleation for the Aβ A21C/A30C peptide withbroken disulphide bonds. It is probable that this is the combined resultof (i) that the coil-like oligomers present in these samples (seeExample 4) have more or less protected disulphide bonds, and (ii) thatmore peptide requires more time to be reduced with TCEP.

Example 3. Wild-Type-Like Structure of Aβ(40) A21C/A30C

SEC was carried out on a Superdex 75 10/300 column (GE Healthcare)equilibrated with 50 mM K⁺ phosphate, 50 mM NaCl, pH 7.2. The flow ratewas 0.8 ml min⁻¹, and the column was operated by an ÅKTA Explorer unit(GE Healthcare). All column runs were carried out at room temperature(21° C.). The column was calibrated with a low molecular-weight gelfiltration calibration kit (Amersham Biosciences) under similarconditions. Typically, 500 μl of peptide was injected into this columnduring each run.

At low concentrations of peptide, e.g. 25 μM, the Aβ(40) A21C/A30Cpeptide typically elutes as an approximately 8-10 kDa protein. Eventhough the Aβ peptide is approximately a 4.5 kDa protein, we do notinterpret this as this peptide is eluting as a dimer because of thepotentially extended structures of unfolded coil-like proteins comparedto the well-folded proteins used in the calibration kit (ribonucelase A,chymotrypsinogen A, ovalbumin, and albumin). In the experiment shown inFIG. 4A, 25 μM of Aβ(40) A21C/A30C peptide was injected into the column,thus generating a single symmetrical peak corresponding to approximatelyan 8 kDa protein. SEC of Aβ(40) wild-type peptide yields identicalresults on this column (not shown), and the apparent size of 8-10 kDa ison the order of what has been demonstrated previously (e.g. in Walsh, D.M., et al., J. Biol. Chem. 272: 22364-22372 (1997)).

A far-UV CD spectrum was collected on a Jasco J-810 spectropolarimeter(Jasco Corporation) at 20° C. using a scan rate of 50 nm min⁻¹. In theexperiment presented in FIG. 4B, 20 μM of Aβ(40) A21C/A30C peptide wasscanned between 190-260 nm in a 1.0 mm cuvette. The spectrum exhibits aminimum around 200 nm that is typical of coil-like structures, and isvery similar to CD spectra of non-aggregated Aβ(40) and Aβ(42) wild typepeptide (not shown; for examples see e.g. Walsh, D. M., et al., J. Biol.Chem. 274: 25945-25952 (1999)).

SDS PAGE was carried out using the Criterion Gel system (Bio-Rad) andprecast 16.5% Tris-Tricine gels (Bio-Rad) at a constant voltage of 120V.The running medium was 100 mM Tris-HCl, 100 mM Tricine, and 0.1% SDS (pH8.3). In FIG. 4C, a 300 μM sample of Aβ(40) A21C/A30C peptide was mixedin a 1:1 ratio with loading buffer (50 mM Tris-HCl, 1% SDS, 20%glycerol, and 0.23% bromphenol blue), and one sample was also made 2.5mM TCEP. The sample containing TCEP was incubated at 95° C. for 5 min,whereas the sample lacking TCEP was incubated at room temperature (21°C.) for approximately 20 min. A volume of 6 μl was then loaded onto thegel together with peptide markers. The gel was run, fixed in 50%methanol and 10% acetic acid (for approximately 1 h), and stained withCoomassie Brilliant Blue for 16 h (0.1% Coomassie, 2.5% 2-propanol, 10%acetic acid). After destaining (5% methanol, 7% acetic acid) the gelswere analyzed. As shown in FIG. 4C, both samples migrate close to the6.2 kDa peptide marker band, thus demonstrating that only anintramolecular disulphide bond has formed in the Aβ(40) A21C/A30Cpeptide.

¹⁵N-HSQC NMR spectroscopy was measured on a Varian Inova 800 MHzspectrometer on ¹⁵N-labelled Aβ(40) A21C/A30C peptide at 50 μM and 1.9mM concentrations of low molecular-weight oligomers (with coilstructure), and on a 50 μM and 450 μM sample of high molecular-weightoligomers (with β-structure). The spectrum in FIG. 4D on 50 μM peptideis indicative of an unstructured coil-like peptide, and is similar topublished spectra of the wild-type Aβ peptide (Hou, L., et al., J. Am.Chem. Soc. 126: 1992-2005 (2003)). Both 50 μM spectra are nearlyidentical (not shown). The 1.9 mM spectrum of low molecular-weightoligomers and that of 450 μM high molecular-weight oligomers were alsoidentical with only a few visible peaks (not shown), indicating thatthese structures are too large for solution NMR. The visible peaks inthe latter two spectra probably stem from the mobile parts of theseoligomers.

Taken together, this example thus demonstrates that the peptidesaccording to the invention have structural properties very similar towild type Aβ peptide despite the presence of an intramoleculardisulphide bond. That the disulphide bond is formed is demonstrated inExamples 1 and 2.

Example 4. SEC Analysis of the Oligomeric Tendencies of the Aβ(40)A21C/A30C

SEC on the Superdex 75 10/300 column was carried out as in Example 3.The larger Superdex 75 16/60 column (GE Healthcare) was operatedsimilarly to the smaller 10/300 column, except that it was run at 1.0 mlmin⁻¹ and that 1 ml was loaded.

FIG. 5 shows one experiment to assay the distribution of oligomers underhigher concentration of peptide than that used in FIG. 4A. The Aβ(40)A21C/A30C peptide was denatured in 6 M guanidinium chloride (GdmCl), 50mM K⁺ phosphate, 50 mM NaCl (pH 7.2), which destroys pre-existingoligomers. GdmCl is a compound routinely used as a protein denaturant. Asolution of approximately 1.8 mM denatured peptide was injected into theSuperdex 75 16/60 column. On the column, the peptide molecules becomeseparated from the GdmCl and are free to oligomerize. Under theseconditions, the Aβ(40) A21C/A30C peptide elutes as a distribution ofsoluble structures. The dominating peak elutes at a position which,according to the protein calibration kit (same as in Example 3),corresponds to approximately a 42 kDa globular protein. Again, webelieve that SEC does not give an accurate determination of Aβ peptidesizes, and these numbers can only be used in a relative sense. CDspectroscopy demonstrated that the oligomers in this peak has coil-likestructure (not shown; the far-UV CD spectrum is virtually identical toFIG. 4B). The distribution of these coil structures ranges down tospecies eluting as approximately 8 kDa proteins. The larger oligomericspecies that elutes as an 85 kDa species has a far-UV CD spectrumconsistent with β structure (not shown; the far-UV CD spectrum of thisfraction is virtually identical to FIG. 7B).

In a related experiment shown in FIG. 6A, a similar concentration ofdenatured Aβ(40) A21C/A30C peptide was loaded onto a Superdex 10/300column. The elution profile exhibits the same features as in FIG. 5,although with lower resolution. As a comparison, SEC on the samedenatured sample first pre-treated with approximately 35 mMβ-mercaptoethanol for 10 min at room temperature (21° C.) andgelfiltrated in the same buffer supplemented with 5 mM TCEP gives almostexclusively high molecular-weight oligomers or protofibrils eluting inthe void volume (>100 kDa) (FIG. 6B). The CD spectrum of this fractionis dominated by β structure (not shown; the far-UV CD spectrum of thisfraction is virtually identical to FIG. 7B).

The top fraction of the oligomeric peak around 10.8 mL in FIG. 6A,having a concentration of 200 μM, was incubated for 4 days at 4° C. andthen gelfiltrated again. The elution profile of this sample in FIG. 6Cshows that a new equilibrium has been established during this time, andthat the species eluting as an 8 kDa protein now dominates. In contrast,pooling all fractions of the oligomeric peak in FIG. 6A after 4 days at4° C. and concentrating this solution to approximately 1.0 mM andreanalyzing it on the same column gives rise to a profile where againlarger oligomeric structures dominate.

Taken together, the SEC experiments demonstrate that by blocking thefibrillation step the Aβ A21C/A30C peptide readily form several types ofoligomers containing different pluralities of monomers. The lowmolecular-weight fractions that elute as >8 kDa to <42 kDa proteins allhave similar coil-like far-UV CD spectra (an example of such a spectrumis shown in FIG. 4B). The high molecular-weight fractions elute as >75kDa proteins and they all have similar far-UV CD spectra consistent withβ-structure (an example of such a spectrum is shown in FIG. 7B).

Example 5. Preparation and Characterization of High Molecular-WeightOligomers of Aβ(40) A21C/A30C

SEC was carried out as in Example 3, with a Sephacryl S-300 26/60 column(GE Healthcare) where 1 ml was injected into the column, which was runat 1.3 ml min⁻¹. The column had been calibrated previously as in Example3. An approximately 1.7 mM Aβ(40) A21C/A30C peptide solution denaturedin 6 M GdmCl, 50 mM K⁺ phosphate, 50 mM NaCl, pH 7.2, was injected intothis larger column and the elution profile is presented in FIG. 7A.Under these conditions, the high molecular-weight fractions elute asβ-structured oligomers at approximately 170 kDa and 80 kDa, whereas thecoil-oligomers elute as smaller proteins.

A far-UV CD spectrum of Aβ(40) A21C/A30C peptide was collected as inExample 3. In the experiment presented in FIG. 7B, 250 μM of Aβ(40)A21C/A30C peptide that eluted from a SEC run at volumes corresponding to75-85 kDa was scanned between 185-260 nm in a 0.1 mm cuvette. Thespectrum exhibits a minimum around 210 nm and positive ellipticityaround 190 nm that is typical of β-like structures of the Aβ(40) andAβ(42) peptides as they oligomerize and aggregate (e.g. Walsh, D. M., etal., J. Biol. Chem. 274: 25945-25952 (1999)). Both the 170 kDa and 80kDa species in FIG. 7A exhibited far-UV CD spectra nearly identical tothe spectrum in FIG. 7B.

Fibrillation assays of 30 μM (monomeric peptide concentration) highmolecular-weight oligomers of the Aβ(40) A21C/A30C peptide was carriedout as in Example 2, with dual samples with and without 10 mM TCEP (FIG.8). During the course of the experiment, there were no fibrils appearingin any of the samples. The baseline is the same as in FIG. 3E.

Example 6. High Molecular-Weight Oligomers of Aβ(40) A21C/A30C Bind theA11 Polyclonal Antibody

Purified rabbit immunoglobulins recognizing a synthetic molecular mimicof soluble Aβ-peptide oligomers (the A11 polyclonal antibody) has beenshown to also recognize similar oligomers in other proteins involved inprotein-misfolding diseases (Kayed, R., et al., Science 300: 486-489(2003)). Since the oligomeric structures are believed to be the toxicspecies in these diseases this suggests a common mechanism ofpathogenesis and, hence, probably also that similar toxic structures areinvolved (Haass, C., and Selkoe, D. J., Nature Reviews Mol. Cell. Biol.8: 101-112 (2007); Glabe, C. G., Trends Biochem. Sci. 29: 542-547(2004)).

FIG. 9 shows a SEC run of a 1 ml Aβ(40) A21C/A30C peptide solution ofisolated and concentrated (to approximately 168 μM) oligomer obtained asin Example 5. SEC was carried out as in Example 3, using a Superdex 7516/60 column (GE Healthcare) run at 1 ml min⁻¹. Under these conditionsthe sample elutes partly in the void volume (approximately 42 ml forthis column, corresponding to >100 kDa proteins) and as highmolecular-weight oligomers (corresponding to approximately 87 kDaproteins). A smaller fraction also elutes as a 7-8 kDa protein.

All fractions of the earlier peak and of the later peak were assayed forA11 (Biosource) binding according to the manufacturer's instructions. 3μl of each fraction was dot blotted on an Immobilion-P^(SQ)polyvinylidene fluoride (PVDF) membrane (Millipore) and allowed to dry.The membrane was then blocked over night (approximately 14 h) in 4° C.in 10% non-fat dry milk dissolved in 20 mM Tris-HCl, 140 mM NaCl, 0.01%Tween 20, pH 7.4 (TBST buffer). The membrane was then washed three timeswith TBST buffer for five minutes, and the A11 primary antibody(Biosource) at 0.5-0.8 μg ml⁻¹ in 5% non-fat dry milk dissolved in TBSTbuffer was applied to the membrane for 1 h at room temperature (21° C.).After washing away unbound antibody three times with TBST buffer forfive minutes, the secondary antibody (a donkey ECL™ anti-rabbit IgGhorseradish peroxidase conjugate from GE Healthcare) at 0.1 μg ml⁻¹ in5% non-fat dry milk dissolved in TBST buffer was added to the membranefor 45-60 min at room temperature (21° C.). After washing away unboundantibody as above, SuperSignal West Pico chemiluminescent substrate(Pierce) was added according to the manufacturer's instructions, andhorseradish peroxidase enzyme activity was detected by a CCD camera(Fujifilm LAS-1000) coupled to a dark box. As shown in FIG. 9, detectionis strongest in the fraction appearing the earliest, thus correspondingto the largest of the high molecular-weight oligomeric species, whereasthe 7-8 kDa peak does not react with the A11 antibody.

In a similar experiment shown in FIG. 10A, 500 μl of an approximately800 μM Aβ(40) A21C/A30C peptide solution in 6 M GdmCl, 50 mM K⁺phosphate, 50 mM NaCl, pH 7.2, was applied to a Superdex 75 10/300column (GE Healthcare), and the fractions blotted as above. Again, theA11 antibody reacts with the larger of the oligomeric species with βstructure. However, it does not recognize the coil oligomers. The firstfraction that had the highest A11-binding capacity containedapproximately 20 μM of Aβ(40) A21C/A30C peptide. This fraction wasincubated for 16 h at 4° C. and then reanalyzed on the same column,yielding the elution profile shown in FIG. 10B which was also assayedfor A11-binding capability. As above, the A11 antibody has a higherreactivity towards the larger of the oligomeric species. FIG. 10B alsodemonstrates that the formation of oligomers with β structure is atleast partly reversible, and that they are relatively stable once formed(in analogy with the aggregation assay in FIG. 8).

Example 7. Coil- to β-Transition in Aβ(40) A21C/A30C

The transition from coil- to β-structure is intimately related to theoligomerization and fibrillization of the wild-type Aβ peptide, and toits toxicity. At solutions of 20-100 μM Aβ peptide at 4° C., thistransition occurs spontaneously within 24 hours for Aβ(42) and days forAβ(40) (Stine, W. B., et al., J. Biol. Chem. 278: 11612-11622 (2003)).For the Aβ(40) A21C/A30C peptide, this structural transition can beaccelerated by heat without fibrillogenesis occurring. FIG. 11A showsthe two CD spectra of a 1.1 mM Aβ(40) A21C/A30C peptide before (coilstructure) and after (β structure) being denatured by a 2° C. min⁻¹gradient of increasing temperature from 20 to 80° C. and back again. Thetransition is shown in FIG. 11B as monitored at 220 nm. In Example 8below, it is demonstrated that the β structures thus formed areprotofibrillar-like. Protofibrils are believed by many to be precursorsof fibrils (e.g. Walsh, D. M., et al., J. Biol. Chem. 274: 25945-25952(1999)). It should be noted that this sample did not exhibit anyamorphous precipitation or fibrils even after this harsh treatment andat this concentration. This resilience towards fibrillogenesis isunprecedented for all derivatives of the Aβ peptide.

Example 8. TEM Analysis of Aβ(40) A21C/A30C Coil- and β-StructuredOligomers, and Protofibrillar-Like Species

TEM images were obtained by a LEO 912 AB OMEGA electron microscope (CarlZeiss SMT AG) equipped with a MegaView CCD camera (Olympus). Negativestaining with uranyl acetate was used in all samples. Formvar/carboncoated nickel grids were activated with UV-light for 5 minutes, afterwhich 5-10 μl of protein sample was applied to each grid for 2 minutes.Two steps of washing with approximately 10 μl of filtered deionized H₂Opreceded the staining. A two-minute treatment with 2% uranyl acetatesolution in filtered deionized H₂O completed the preparation of thegrids, which were allowed to air dry for a few minutes before storage orimmediate analysis.

In FIG. 12A, an approximately 500 μM solution containing Aβ(40)A21C/A30C in coil conformation was applied to the grids. In FIG. 12B,the same solution used to prepare the sample in FIG. 12A was denaturedat 60° C. for 20 minutes prior to being applied to the grid. In FIG.12C, an approximately 150 μM solution of Aβ(40) A21C/A30C oligomers withβ structure (obtained as in Example 5) was analyzed.

The scale bar is 200 nm in FIG. 12. The spherical structures observed inFIG. 12A and FIG. 12C are similar in morphology and dimension to eachother, having diameters of 15 nm to 35 nm. They are also similar topreviously published TEM images that have used the same stainingtechnique. These spherical structures of wild-type Aβ(40) peptide wererecently observed for oligomers of Aβ(40) having β structure (Chimon,S., et al., Nature Struct. Mol. Biol. 14: 1157-1164 (2007)). In thispublication, Chimon et al. claim that these structures are the toxicspecies.

The structures observed in FIG. 12B are protofibrillar-like, with anaverage dimension of 6-7 nm in width and approximately 36 nm in length(although the lengths vary). They are very similar in morphology anddimension to the wild-type Aβ(40) protofibrils, which have averagewidths of 6-10 nm and lengths of 5-160 nm (Walsh, D. M., et al., J.Biol. Chem. 272: 22364-22372 (1997)).

Example 9. Fibrillation Assay of Aβ(42) A21C/A30C

The fibrillation assay was carried out as in example 2 with theexception that data points were recorded every 10 min. The concentrationof Aβ(42) A21C/A30C was approximately 117 μM. The result is presented inFIG. 13.

Fibrillation of the reduced sample commences immediately, whereas thetwo oxidized samples remain non-fibrillized throughout the 42-h assay.This experiment demonstrates that the A21C/A30C disulphide bond iseffective in preventing fibrillation also in the Aβ(42) derivative,which has a much higher inherent tendency to fibrillate compared to theAβ(40) peptide.

Example 10. SEC Analysis of the Aβ(42) A21C/A30C with a Dot Blot of theDifferent Fractions Against the A11 Polyclonal Antibody

The SEC was carried out on a Sephacryl S300 16/60 column as in example5, and the dot blot was carried out as in example 6. The result ispresented in FIG. 14.

Comparing the chromatogram in FIG. 14 with the chromatogram in FIG. 7Afor the Aβ(40) A21C/A30C peptide demonstrates that the Aβ(42) A21C/A30Cpeptide has a higher tendency to oligomerize into high molecular-weightoligomers. But the oligomeric forms of both Aβ(42) A21C/A30C and Aβ(40)A21C/A30C peptides have similar sizes. The A11-binding species elutes inthe void on this column which has a cutoff of approximately ≥1500 kDa.

Example 11. Stability of the A11-Binding Epitope

A 80-kDa fraction of Aβ(40) A21C/A30C peptide obtained as in example 5was concentrated to 20 μM and incubated at 37° C. in 50 mM K⁺ phosphatebuffer, 50 mM NaCl, and 0.05% azide. Samples were removed at regularintervals during a two-month period and stored at −24° C. untilanalysis. The dot blot was carried out as in example 6, and the resultis presented in FIG. 15A. The A11 epitope formed quickly already duringthe concentration step, and remained stable during the full course ofthe experiment (for approximately 60 days). The fluorescence intensitydecreased with only 0.1% per day.

A related experiment on a monomeric fraction of Aβ(40) A21C/A30C peptideobtained from the same SEC as above was concentrated to 90 μM andincubated at 37° C. in 50 mM K⁺ phosphate buffer, 50 mM NaCl, and 0.05%azide. Samples removed at the same intervals as above during a two-monthperiod did not exhibit any A11 binding (not shown).

A similar sample of monomeric Aβ(42) A21C/A30C peptide obtained as inexample 5 and concentrated to 50 μM was incubated at 37° C. in 50 mM K⁺phosphate buffer, 50 mM NaCl, and 0.05% azide was probed for A11binding. This sample displayed the A11 epitope already after a few days(FIG. 15B). Once nucleated, the oligomerization into the high molecularweight oligomers occurred with a t_(1/2) of 2.2 days in this experiment.

Taken together, these experiments demonstrate that the high molecularweight oligomers are formed spontaneously in the Aβ(42) A21C/A30Cpeptide whereas the shorter Aβ(40) A21C/A30C peptide is more resilienttowards the conformational change into β structure that is intimatelyassociated with oligomerization into the high molecular-weightoligomers. Once formed, the A11 binding oligomers are remarkably stable,again demonstrating the ability of the invention to arrest thefibrillation step completely.

The invention claimed is:
 1. A polypeptide, which is an amyloidprecursor protein (APP), wherein i) the amino acid alanine in position692, as defined by the amino acid sequence of the human APP in SEQ IDNO: 12 is replaced with a cysteine; and ii) the amino acid alanine inposition 701, as defined by said amino acid sequence of said human APPin SEQ ID NO: 12 is replaced with a cysteine, wherein cleavage of saidpolypeptide derives a peptide fragment comprising the amino acidsequence L-V-F-F-C (SEQ ID NO: 13) corresponding to amino acids 17 to 21of SEQ ID NO: 4 and the amino acid sequence C-I-I-G-L-M-V (SEQ ID NO:14) corresponding to amino acids 30 to 36 of SEQ ID NO: 4, said peptidefragment comprising: exactly two cysteines corresponding to Cys21 andCys30 in SEQ ID NO: 4; and exactly one disulfide bond, said exactly onedisulfide bond is between said two cysteines corresponding to Cys21 andCys30 in SEQ ID NO: 4, wherein said peptide fragment is resistanttowards fibrillogenesis.
 2. The polypeptide according to claim 1,wherein said peptide fragment comprises the amino acid sequenceL-V-F-F-C-E-D-V-G-S-N-K-G-C-I-I-G-L-M-V (SEQ ID NO: 16) corresponding toamino acids 17 to 36 of SEQ ID NO:
 4. 3. The polypeptide according toclaim 2, wherein said peptide fragment comprises the amino acid sequencecorresponding to amino acids 1 to 40 of SEQ ID NO:
 4. 4. The polypeptideaccording to claim 3, wherein said peptide fragment comprises the aminoacid sequence SEQ ID NO:
 4. 5. The polypeptide according to claim 1consisting of the amino acid sequence in SEQ ID NO:
 11. 6. Thepolypeptide according to claim 1, wherein said two cysteinescorresponding to Cys21 and Cys30 in SEQ ID NO: 4 correspond to Cys692and Cys701 in SEQ ID NO: 12.